1. Field of Invention
The present invention relates to an etching apparatus which is suitable for manufacturing an optical device with a membrane (e.g., an X-ray transmission device or transmission filter), and to a method for manufacturing an optical devices having a membrane.
2. Description of the Related Art
As an X-ray filtering optical device, zone plates are broadly used. A typical zone plate is shown in FIG. 4, which has a membrane 32 having a thickness of 1 micron or less, and a circular diffraction grating pattern 33' which is made of an X-ray absorbing material 33 and positioned on the membrane 32.
To support the membrane 32, a silicon wafer (substrate) 31 having a thickness of several hundred micron is generally used. A general method for manufacturing such a zone plate comprises the steps of forming silicon nitride films 32' and 35' on the top and bottom surfaces of the substrate 31 by vacuum deposition up to a thickness of about 0.1 micron, and etching the bottom silicon nitride film 35' by, for example, an RIE method, to complete a silicon nitride film 35 having an aperture 34.
The membrane 32 is a part of the top silicon nitride film 32' and positioned directly above the aperture 34. An example of the vacuum deposition for forming the silicon nitride films 32' and 35' is a low-pressure chemical vapor deposition (LPCVD) method. In order to efficiently control the stress of the membrane, the stoichiometric ratio of Si to N is set so that Si is slightly rich.
The pattern 33' is formed of an X-ray absorbing material on the silicon nitride film 32'. Examples of the X-ray absorbing material include gold, tungsten, tantalum, chromium, nickel, and any other materials that have very low transmissivities to X-rays at the wavelength in use and are easily micropatterned. The pattern 33' can be formed by, for example, an RIE method, a lift-off method, plating, or electron-beam lithography.
Then, the silicon substrate 31 is etched from the aperture 34 of the bottom silicon nitride film 35 so that the bottom face of the membrane 32 is exposed above the aperture 34. Preferably, the silicon substrate 31 is etched by wet etching using ammonia solution or potassium hydroxide solution as an etchant.
Potassium hydroxide solution is corrosive to monocrystalline silicon, but is not corrosive to silicon nitride. Taking advantage of this property, the silicon nitride film 35 functions as the mask for etching the silicon substrate 31.
When the substrate 31, which is covered with the silicon nitride films 32' and 35, is immersed in the slightly heated etchant, the silicon substrate 31 is etched only from the aperture 34 until reaching the top silicon nitride film 32' which remains as the membrane 32 without being etched.
The etching speed of the potassium hydroxide solution differs to a great extent depending on the surfaces with respect to the crystallographic axis of the silicon substrate 31. In particular, the silicon substrate 31 is hardly etched in the lateral direction, while it is easily etched in the vertical direction in FIG. 4. Accordingly, the dimensions of the membrane 32 becomes slightly smaller than those of the aperture 34.
There are many other membrane optical devices other than zone plates. For example, various types of X-ray filters, X-ray transmission masks, electron-beam exposure masks are known. These devices are all manufactured by etching in the similar manner as the zone plates.
The etching rate of silicon slightly varies depending on the type or the temperature of etchant used; however, it is generally in the range from several micrometer per minute to several tens micrometer per minute. In order to completely etch a silicon board having a thickness of several hundred micrometer to several millimeter, it takes about 5-12 hours.
In this situation, a conventional etching apparatus achieves only a few etching cycles a day, and the yield of a conventional etching apparatus for manufacturing optical devices is very low.
Since membrane optical devices fabricated by etching are broadly used in various fields, the sizes and the shapes of the membrane optical devices differ depending on their purpose and use. Naturally, the sizes and the shapes of the base materials (i.e., the substrates) which are to be etched in the manufacturing process also differ depending on the use.
Conventionally, when a novel membrane device is conceived and developed, a new etching apparatus is designed according to the size and the shape of the new device. To this end, certain amount of time and expense are required before the new device is actually manufactured. This implies that each etching apparatus is not efficiently used and that the manufacturing cost of optical devices is increased.
Another problem is that if wet etching is performed using an etchant in the conventional etching apparatus, the X-ray absorber pattern 33' formed on the silicon nitride film 32' often peels off, or it is corroded by the etchant.
For example, it takes 5-6 hours to etch a silicon substrate having a thickness of 400 mm. In this case, most of the X-ray absorber pattern peels off from the silicon nitride film (i.e., the membrane) 32 on the substrate through the etching process. In addition, the membrane may irreversibly deform or break during the etching process using the conventional etching apparatus.